ch-17.pptx - Web viewCAMPBELLBIOLOGY EDITIONTENTHReece • Urry • Cain • Wasserman • Minorsky • Jackson17Gene Expression: From Gene to ProteinLecture Presentation byNicole

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CAMPBELL

BIOLOGY EDITION

TENTH

Reece • Urry • Cain • Wasserman • Minorsky • Jackson

17Gene Expression: From Gene to Protein

Lecture Presentation byNicole Tunbridge and Kathleen Fitzpatrick

© 2014 Pearson Education, Inc.

The Flow of Genetic Information

The information content of genes is in the specificsequences of nucleotides The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins Proteins are the links between genotype and phenotype Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation


Evidence from the Study of Metabolic Defects

In 1902, British physician Archibald Garrod firstsuggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme Cells synthesize and degrade molecules in a series of steps, a metabolic pathway


Basic Principles of Transcription and Translation RNA is the bridge between genes and the proteins for which they code Transcription is the synthesis of RNA using information in DNA Transcription produces messenger RNA (mRNA)

Translation is the synthesis of a polypeptide, using information in the mRNA Ribosomes are the sites of translation


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Figure 17.3

Nuclearenvelope

TRANSCRIPTION DNA

Pre-mRNA RNA PROCESSING

NUCLEUS mRNA

DNACYTOPLASM

CYTOPLASM mRNARibosome Ribosome

TRANSLATION

Polypeptide Polypeptide

(a) Bacterial cell (b) Eukaryotic cell


TRANSCRIPTION

TRANSLATION

A primary transcript is the initial RNA transcriptfrom any gene prior to processing The central dogma is the concept that cells are governed by a cellular chain of command: DNA → RNA → protein


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Figure 17.UN01

DNA RNA Protein


The Genetic Code

How are the instructions for assembling aminoacids into proteins encoded into DNA? There are 20 amino acids, but there are only four nucleotide bases in DNA How many nucleotides correspond to an amino acid?


Codons: Triplets of Nucleotides

The flow of information from gene to protein isbased on a triplet code: a series of nonoverlapping, three-nucleotide words The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA These words are then translated into a chain of amino acids, forming a polypeptide


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Figure 17.4

DNAtemplate strand3′ 5′

A C C A A A C C G A G T

5′T G G T T T G G C T C A 3′

TRANSCRIPTION

mRNA 5′ U G G U U U G G C U C A 3′

Codon TRANSLATION

Protein Trp Phe Gly Ser

Amino acid© 2014 Pearson Education, Inc.

During transcription, one of the two DNA strands,called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript The template strand is always the same strand for a given gene


During translation, the mRNA base triplets, calledcodons, are read in the 5′ → 3′ direction Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide


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Cracking the Code

All 64 codons were deciphered by the mid-1960s Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation The genetic code is redundant (more than one codon may specify a particular amino acid) but not ambiguous; no codon specifies more than one amino acid Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced© 2014 Pearson Education, Inc.

Figure 17.5Second mRNA base

U C A G

UUUPhe

UCUUCCUCA UCG

UAUTyr

UAC

UGUUGC Cys

U UUCUUA UUG Ser

Leu

UC A

UAG Stop UGG Trp G

CUU

Leu

CCUCCCCCA CCGPro

CAUCACCAA CAG

HisCUCCCUACUG

CGUCGC CGA CGG

Arg

Gln

UCA G

AUUAUCAAUA

ACUACC

ThrACAACG

AAUAAC AAAAAG

Asn

Lys

AGUAGCAGA AGG

SerIle

Arg

UCA G

GUU GCUGCCGCA GCG

GAUGAC GAAGAG

AspG

GUCGUA GUGVal Ala

GGUGGC GGA GGG

GlyGlu

UCA G


UAA Stop

AUG Met orstart

UGA Stop

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Firs

t mR

NA b

ase

(5′ e

nd o

f

Third

mR

NA b

ase

(3′ e

nd o

f

Concept 17.2: Transcription is the DNA-directed synthesis of RNA: A closer look Transcription is the first stage of gene expression


Molecular Components of Transcription

RNA synthesis is catalyzed by RNA polymerase,which pries the DNA strands apart and joins together the RNA nucleotides The RNA is complementary to the DNA template strand RNA polymerase does not need any primer

RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine


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Figure 17.7-3PromoterTranscription unit

5′3′

3′5′

Start pointRNA polymerase

Initiation

5′3′

Unwound DNAElongation

RNAtranscript

3′5′

Template strand of DNA

5′3′

RewoundDNA

3′ 3′5′

5′RNAtranscript

Direction of transcription

3 Termination(“downstream”)

5′3′

3′5′

5′3′Completed RNA transcript


The DNA sequence where RNA polymeraseattaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator The stretch of DNA that is transcribed is called atranscription unit


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Elongation of the RNA Strand

As RNA polymerase moves along the DNA, ituntwists the double helix, 10 to 20 bases at a time Transcription progresses at a rate of 40 nucleotides per second in eukaryotes A gene can be transcribed simultaneously by several RNA polymerases Nucleotides are added to the 3′ end of the growing RNA molecule


Figure 17.9

Nontemplatestrand of DNARNA nucleotidesRNApolymerase

A TA C C A3′ 5′

3′ endU

A U C C A

5′ 3′T A G G T T

5′ Direction of transcriptionTemplatestrand of DNA

Newly madeRNA


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Figure 17.13

GeneDNA

Exon 1 Intron Exon 2 Intron Exon 3

Transcription RNA processingTranslation

Domain 3

Domain 2

Domain 1

Polypeptide© 2014 Pearson Education, Inc.

Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: A closer look Genetic information flows from mRNA to protein through the process of translation


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Molecular Components of Translation

A cell translates an mRNA message into proteinwith the help of transfer RNA (tRNA) tRNAs transfer amino acids to the growing polypeptide in a ribosome Translation is a complex process in terms of its biochemistry and mechanics


Figure 17.14

PolypeptideAmino acids

tRNA with amino acid attached

Ribosome

Gly

tRNA

AnticodonA A A

U G G U U U G G C5′mRNA

Codons 3′


The Structure and Function of Transfer RNA

Molecules of tRNA are not identical Each carries a specific amino acid on one end

Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA


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A tRNA molecule consists of a single RNA strandthat is only about 80 nucleotides long Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf


Because of hydrogen bonds, tRNA actually twistsand folds into a three-dimensional molecule tRNA is roughly L-shaped


Figure 17.15

3′ Amino acid A

attachment C

site C

A 5′C GG CC GU GU AA UA U

Amino acid attachment site

5′3′

U* GC

CC A C A G U A G

G U G U *A * C U CC G A G

** G

* *CU* G A

*G A G Hydrogen bonds

G CG C Hydrogen

U A bondsGAAC* A

A

U G A A G3′5′

Anticodon(a) Two-dimensional structure

Anticodon Anticodon

(b) Three-dimensional structure(c) Symbol used in this book


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Accurate translation requires two steps First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase Second: a correct match between the tRNA anticodon and an mRNA codon

Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon


Figure 17.16-31 Amino acid and tRNA enter active site.Tyrosine (Tyr) (amino acid)

Tyrosyl-tRNA synthetase

Tyr-tRNA

A U A ATP

Aminoacyl-tRNAsynthetase

Complementary tRNA anticodonAMP + 2 P i tRNA

3 Aminoacyl tRNAreleased.

2 Using ATP, synthetase catalyzes covalent bonding.Aminoacid

Computer model© 2014 Pearson Education, Inc.

Ribosomes

Ribosomes facilitate specific coupling of tRNAanticodons with mRNA codons in protein synthesis The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA) Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences: some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosomes


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Figure 17.17a

tRNAmolecules

Growingpolypeptide

Exit tunnel

E

Largesubunit

P A

Smallsubunit

5′mRNA 3′

(a) Computer model of functioning ribosome


Figure 17.17b

P site (Peptidyl-tRNA binding site)Exit tunnel

E site(Exit site)

A site (Aminoacyl-tRNA binding site)

E P A Largesubunit

mRNAbinding site Small

subunit

(b) Schematic model showing binding sites


Figure 17.17c

Amino end

Growingpolypeptide

Next amino acid to be added to polypeptidechain

E tRNAmRNA 3′

5′ Codons

(c) Schematic model with mRNA and tRNA


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A ribosome has three binding sites for tRNA The P site holds the tRNA that carries the growing polypeptide chain The A site holds the tRNA that carries the next amino acid to be added to the chain The E site is the exit site, where discharged tRNAs leave the ribosome


Building a Polypeptide

The three stages of translation Initiation

Elongation

Termination All three stages require protein “factors” that aid in the translation process Energy is required for some steps also


Ribosome Association and Initiation of Translation Initiation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits First, a small ribosomal subunit binds with mRNA and a special initiator tRNA Then the small subunit moves along the mRNA until it reaches the start codon (AUG) Proteins called initiation factors bring in the large subunit that completes the translation initiation complex


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Figure 17.18

3′ U A C 5′ P site5′ A G 3′

Large ribosomalsubunit

U

InitiatortRNA

P i+

GTP GDP

mRNA E A

5′

mRNA binding site

3′Small ribosomal subunit

5′ 3′Start codon

Translation initiation complex

1 Small ribosomal subunit bindsto mRNA.

2 Large ribosomal subunitcompletes the initiation complex.


Elongation of the Polypeptide Chain

During elongation, amino acids are added oneby one to the C-terminus of the growing chain Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation Energy expenditure occurs in the first and third steps Translation proceeds along the mRNA in a 5′ → 3′ direction© 2014 Pearson Education, Inc.

Figure 17.19-1Amino endof polypeptide

E 3′mRNA

1 Codon recognition

5′P A

site site GTP

GDP + P i

E

P A


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E 3′mRNA

1 Codon recognition

5′P A

site site GTP

GDP + P i

E

P A

2 Peptide bondformation

E

P A



E 3′mRNA

1 Codon recognition

Ribosome ready fornext aminoacyl tRNA 5′ P A

site site GTP

GDP + P i

E E

P A P A

3 Translocation

GDP + P iGTP

2 Peptide bond formation

E

P A


Termination of Translation

Termination occurs when a stop codon in themRNA reaches the A site of the ribosome

The A site accepts a protein called a release factor

The release factor causes the addition of a water molecule instead of an amino acid This reaction releases the polypeptide, and the translation assembly comes apart


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Figure 17.20-3

Releasefactor

Freepolypeptide

5′

3′3′

5′ 5′ 3′2 GTP

Stop codon(UAG, UAA, or UGA)

1 Ribosome reaches a stop codon on mRNA.

2 Release factor

2 GDP + 2 P i3 Ribosomal subunits

promotes hydrolysis.and other componentsdissociate.


Figure 17.22Growingpolypeptides

Completedpolypeptide

Incomingribosomal subunits

Start of mRNA(5′ end) End of

mRNA(3′ end)

(a) Several ribosomes simultaneously translatingone mRNA molecule

RibosomesmRNA

(b) A large polyribosome in a bacterial 0.1 µmcell (TEM)


Making Multiple Polypeptides in Bacteria and Eukaryotes Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome) Polyribosomes enable a cell to make many copies of a polypeptide very quickly


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Concept 17.5: Mutations of one or a few nucleotides can affect protein structure and function Mutations are changes in the genetic material of a cell or virus Point mutations are chemical changes in just one base pair of a gene The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein


If a mutation has an adverse effect on thephenotype of the organism the condition is referred to as a genetic disorder or hereditary disease


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Figure 17.25

Wild-type β-globin Sickle-cell β-globin

Wild-type β-globin DNAMutant β-globin DNA3′C T C5′ 3′C A C5′5′G A G3′ 5′G T G3′

mRNA mRNA5′G A G3′ 5′G U G3′

Normal hemoglobinGlu

Sickle-cell hemoglobinVal


Types of Small-Scale Mutations

Point mutations within a gene can be divided intotwo general categories Nucleotide-pair substitutions

One or more nucleotide-pair insertions or deletions


Substitutions

A nucleotide-pair substitution replaces onenucleotide and its partner with another pair of nucleotides Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code Missense mutations still code for an amino acid, but not the correct amino acid Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein© 2014 Pearson Education, Inc.

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Figure 17.26a

Wild typeDNA template strand 3′ T A C T

T C A A A C C G A T T 5′5′ A T G A A G T T T G G C T A A 3′

mRNAProtein Amino end

5′ A U G A A G U U U G G C U A A 3′Met Lys Phe Gly Stop

Carboxyl end

Nucleotide-pair substitution: silentA instead of G

3′ T A C T T C A A A C C A A T T 5′5′ A T G A A G T T T G G T T A A 3′

U instead of C5′ A U G A A G U U U G G U U A A 3′

MetLysPheGly Stop


Insertions and Deletions

Insertions and deletions are additions or lossesof nucleotide pairs in a gene These mutations have a disastrous effect on the resulting protein more often than substitutions do Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation


Figure 17.26b

Wild typeDNA template strand 3′ T A C T

T C A A A C C G A T T 5′5′ A T G A A G T T T G G C T A A 3′

mRNAProtein Amino end

5′ A U G A A G U U U G G C U A A 3′Met LysPhe Gly Stop

Carboxyl end

Nucleotide-pair substitution: missenseT instead of C3′ T A C T T C A A A T C G A T T 5′5′ A T G A A G T T T A G C T A A 3′A instead of G5′ A U G A A G U U U A G C U A A 3′

MetLysPheSer Stop


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New Mutations and Mutagens

Spontaneous mutations can occur during DNAreplication, recombination, or repair Mutagens are physical or chemical agents that can cause mutations


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Documents

ch-17.pptx - Web viewCAMPBELLBIOLOGY EDITIONTENTHReece • Urry • Cain • Wasserman • Minorsky • Jackson17Gene Expression: From Gene to ProteinLecture Presentation byNicole